Semiconductor devices and methods to form trenches in semiconductor devices

ABSTRACT

Semiconductor devices and methods of fabricating the same are disclosed. One example method may include forming sequentially a pad oxide film and a silicon nitride film on an entire surface of a semiconductor substrate, forming the trench by etching the silicon nitride film and the semiconductor substrate up to a predetermined depth, and forming a liner oxide film with a thickness thinner than that of the silicon nitride film on an inner wall of the trench. The example method may also include applying a negative voltage to a back surface of the semiconductor substrate and forming an insulation film to fill the trench on the liner oxide film.

TECHNICAL FIELD

The present disclosure relates to semiconductors and, more particularly,to semiconductor devices and methods to form trenches in semiconductordevices.

BACKGROUND

Shallow trench isolation (STI) structures have been widely used toisolate areas within semiconductor devices. These STI structures areadvantageous to miniaturization of semiconductor devices because a sizeof a field region is limited to a desired size of a trench by formingtrenches in a semiconductor substrate and filling the trenches withinsulation material.

Hereinafter, a conventional method of manufacturing the STI structurewill be in brief described. FIGS. 1 a to 1 d are sectional views showingthe conventional STI manufacture method.

First, as shown in FIG. 1 a, a pad oxide film 2 is deposited at athickness of about 200 Å on an entire surface of a silicon substrate 1.Subsequently, a silicon nitride film 3 is deposited at a thickness ofabout 2000 Å on the pad oxide film 2 and a photosensitive film isapplied and exposed on the silicon nitride film 3. A pattern ofphotosensitive film 4 is then formed by removing only the photosensitivefilm on a region that is to include a trench.

Next, as shown in FIG. 1 b, a trench 100 is formed in the siliconsubstrate I by dry etching the exposed silicon nitride film 3, the padoxide film 2, and the silicon substrate 1 up to a predetermined depthusing the pattern of photosensitive film 4 as a mask. The pattern ofphotosensitive film 4 is removed and then a cleaning process isperformed.

As shown, an edge at which a side and a bottom of the formed trenchintersect forms a nearly right angle. It is almost impossible todecrease this angle so that the edge is gently slanted.

Next, as shown in FIG. 1 c, a liner oxide film 5 is formed at an innerwall of the trench 100 using a thermal oxidation process. According toone example, the liner oxide film 5 is formed at about 60% of its totalthickness inside the silicon substrate 1 and at about 40% of the totalthickness outside the silicon substrate 1 by a typical thermal oxidationprocess. The liner oxide file 5 is centered at a surface (shown as adotted line in FIG. 1 c) of the silicon substrate 1 of the trench.

During the thermal oxidation process for the formation of the lineroxide film 5, as an angle of an edge at which a side and a bottom of thetrench 100 intersect becomes smaller, it becomes difficult for oxygenmolecules to penetrate into the silicon substrate. For example, a nearlyvertical trench edge creates a state in which oxygen molecules do noteasily penetrate into the silicon substrate.

At this time, because most of deposition processes are performed usingonly heat in a high temperature without any electric power, the entiresurface of the silicone substrate 1 assumes electrical neutrality. Underthis state, as shown in FIG. 1 d, a field oxide 6 is thickly depositedon an entire surface of the silicon nitride film 3 including the lineroxide film 5 such that the trench 100 is sufficiently buried or filled.

The field oxide 6 is consecutively deposited at the same speed on thesilicon nitride film 3 or in the interior of the trench 100 on the lineroxide film 5 with a surface state as shown as a dotted line in FIG. 1 d.During the deposition process, the field oxide 6 may create a shape thatis difficult to fill. Accordingly, a void 200 in the field oxide 6 maybe created in the interior of the trench 100. If this void 200 isexcessively large, the void 200 will be exposed when a chemicalmechanical polishing is performed to planarize the filed oxide 6. Theexposing of the void 200 results in difficulties during theplanarization process.

In addition, in a state where the void is exposed after theplanarization, when a polysilicon to be deposited for formation of anelectrode in a subsequent process enters the void, a leakage current mayresult, thereby causing erroneous operation of a device and acircuit-short between adjacent devices. These effects are fatal to theoperation of the device.

The above problems become more serious as a width of the trench becomesnarrower.

One prior approach to filling the trench without any void is disclosedin Korean Patent No. 36355, which discloses a technique by which acomposite film structure of an anti-diffusion insulation film and athermal oxide film is provided between a nitride film liner and a trenchin order to minimize a transistor characteristic deterioration due tothe nitride film liner. However, this approach has a disadvantage inthat a manufacturing process for forming the composite film structure iscomplicated.

Another approach is disclosed in Korean Patent Application No.2003-1409, which discloses techniques by which a first liner oxide filmformed in an inner wall of a trench is etched away by a wet etchingmethod, a second liner oxide film is thermally grown such that a topsurface of the second liner oxide film has a smoothly curved edge, andthen a filed oxide is formed on the top surface of the second lineroxide film such that the trench is completely filled without any void.However, because this technique requires a process of wet etching theliner oxide film and two deposition processes, this second priorapproach also has a disadvantage of a complicated manufacturing process.

Still other prior approaches are disclosed in U.S. Pat. Nos. 6,521,413and 6,214,698, which disclose techniques by which an undoped thin filmis used for preventing voids, or a gap fill is achieved by twoprocesses. However, since these techniques require a modification ofprocess conditions, it also has a disadvantage of a complicatedmanufacturing process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a to 1 d are sectional views showing the results of a method offorming a trench in a conventional semiconductor device.

FIGS. 2 a to 2 f are sectional views showing results of a disclosedexample method of forming a trench in a semiconductor device.

DETAILED DESCRIPTION

First, as shown in FIG. 2 a, a pad oxide film 12 is thinly deposited onan entire surface of a silicon substrate 12, and a silicon nitride film13 is deposited on the pad oxide film 12. A photosensitive film 14 isthen applied and exposed on the silicon nitride film 13, and then apattern of photosensitive film 14 is formed by removing only thephotosensitive film on a region to be formed with a trench.

At this time, the pad oxide film 12 is optionally deposited to prevent astress of the silicon nitride film 13 from being transferred to thesemiconductor substrate 11. In one example, the pad oxide film 12 isdeposited thinly at a thickness of between about 100-300 Å, for example,200 Å.

Because the silicon nitride film 13 is made of material having a highselectivity over the pad oxide film, it functions as a buffer layer in asubsequent chemical mechanical polishing process for the field oxide. Inone example, the silicon nitride film 13 is deposited at a thickness of1000-3000 Å, for example, 2000 Å.

Next, as shown in FIG. 2 b, a trench 100 is formed in the semiconductorsubstrate 11 by dry etching the exposed silicon nitride film 13, the padoxide film 12, and the semiconductor substrate 11 to a predetermineddepth using the pattern of photosensitive film 14 as a mask.Subsequently, the pattern of photosensitive film 14 is removed and acleaning process is performed.

At this time, because an etching rate of the silicon nitride film 13 isdifferent from an etching rate of the semiconductor substrate 11,process conditions for a dry etching of the silicon nitride film 13 aredifferent from those for a dry etching of the semiconductor substrate11. Accordingly, the dry etching may be performed in two steps. In moredetail, after the silicon nitride 13 is first etched away, thesemiconductor substrate 11 is etched away up to a predetermined depthwith etching process conditions different from those of the siliconnitride film 13. When etching of the silicon nitride film 13 and etchingof the semiconductor substrate 11 are consecutively performed withdifferent process conditions, as described above, etching process timecan be shortened.

The trench 100 formed by the etching process has an angled edge at whicha side and a bottom of the trench intersect. It is almost impossible toform this edge into a gently slanted shape.

Next, as shown in FIG. 2 c, a liner oxide film 15 is formed at an innerwall of the trench 100 using a thermal oxidation process. In oneexample, the liner oxide film 15 is formed at about 60% of its totalthickness inside the silicon substrate 11 and at about 40% of the totalthickness outside the silicon substrate 11 by a typical thermaldiffusion process, centering at a surface (shown as a dotted line inFIG. 2 c) of the semiconductor substrate 11 of the trench. The lineroxide film 15 may be deposited at a thickness of 100-500 Å, for example,300 Å.

Next, as shown in FIG. 2 d, a negative (−) voltage of −2000V to −1000Vis applied to a back surface of the semiconductor substrate 11. Anelectrostatic chuck (ESC) 20 can be used for this voltage application,however it is not limited thereto. For example, the negative voltage maybe directly applied using any electrode. As another method of applyingthe negative voltage, as shown in FIG. 2 e, electrons can be injectedinto the back surface of the semiconductor substrate 11 using anelectron gun 20. According to one example, for the electron injection, avoltage of −2000V to −1000V is applied to the back surface of thesemiconductor substrate 11.

When the negative voltage is applied to the back surface of thesemiconductor substrate, as described above, positive (+) charges arestored in the front surface of the semiconductor substrate 11. At thistime, the positive charges stored in the liner oxide film 15 are morethan those stored in the silicon nitride film 13, which is thicker thanthe liner oxide film 15.

This can be seen from the following mathematical expression 1 showingthat an electrostatic capacity C is proportional to an area A of adielectric and is inversely proportional to a thickness d of thedielectric.C=(A/d)×ε  (1)

Where, ε means a dielectric constant. Dielectric constants of the lineroxide film 15 and the silicon nitride film 13 are 4.3 and 7.2,respectively. A thickness of the liner oxide film 15 is about 300 Å anda thickness of the silicon nitride film 13 is about 2000 Å. Accordingly,charges stored in the relatively thin liner oxide film 15 are even morethan those stored in the relatively thick silicon nitride film 13.

Next, as shown in FIG. 2 f, a field oxide 16 is thickly deposited on anentire surface of the liner oxide film 15 and the silicon nitride film13 such that the trench is sufficiently filled. The field oxide 16 isformed by an atmospheric pressure chemical vapor deposition (APCVD)method or a sub-atmospheric chemical vapor deposition (SACVD) method. Inthis course, reactive gas particles for deposition assume negative (−)charges through decomposition and generation steps, and are electricallyand chemically combined to the substrate assuming positive (+) charges,resulting in the deposition of the field oxide 16.

Because positive (+) charges stored in the liner oxide film 15 are evenmore than those stored in the silicon nitride film 13, the field oxide16 is still lively deposited on the liner oxide film 15. In other words,a speed of deposition of the field oxide 16 in the trench 100 is higherthan that on the silicon nitride film 13. As a result, the trench 100can be completely filled with the field oxide 16 without any void.

Finally, the trench isolation process may be completed by chemicalmechanical polishing and planarizing the field oxide 16 until thesilicon nitride film 13 is exposed.

As described above, by injecting electrons into the back surface of thesemiconductor substrate using an electron gun, positive charges arestored in the liner oxide film and the silicon nitride film located onthe front surface of the semiconductor substrate such that positivecharges are more stored in the liner oxide film and are less stored inthe silicon nitride film thicker than the liner oxide film. Under thisstate, because the field oxide for filling the trench is deposited onthe entire surface the liner oxide film and the silicon nitride film,the speed of deposition of the field oxide in the trench is higher thanthat on the silicon nitride film. Accordingly, the trench can becompletely filled with the filled oxide without any void.

Accordingly, the deterioration of the reliability of device due to aleakage current or circuit short by voids can be prevented and thedevice yield can be improved.

As disclosed herein, example semiconductor device manufacturing methodsfill a trench formed as a field region to isolate one active region fromanother in a semiconductor device with insulation material without anyvoid. In particular, an example trench formation method is characterizedin that electrons are scanned to a back surface of a semiconductorsubstrate by an electron gun and a trench is filled with an insulationfilm under a state where positive charges are more stored in arelatively thin liner oxide film and are less stored in a relativelythick silicon nitride film.

One example method includes forming a silicon nitride film on an entiresurface of a semiconductor substrate, forming the trench by etching thesilicon nitride film and the semiconductor substrate up to apredetermined depth, forming a liner oxide film with a thickness thinnerthan that of the silicon nitride film on an inner wall of the trench,applying a negative voltage to a back surface of the semiconductorsubstrate, and forming an insulation film to fill the trench on theliner oxide film.

In one example, the silicon nitride film is formed at a thickness of1000-3000 Å, and the liner oxide film is formed at a thickness of100-500 Å by a thermal oxidation process. By way of further example, theapplication of the negative voltage includes applying a voltage of−2000V to −1000V to the back surface of the semiconductor substrate. Thevoltage application may be accomplished by applying the negative voltageto the back surface of the semiconductor substrate using anelectrostatic chuck (ESC) or by injecting electrons into the backsurface of the semiconductor substrate using an electron gun.

In one example, forming the insulation film may include forming afilling oxide film to fill the trench on an entire surface of thesilicon nitride film and then chemical mechanical polishing the oxidefilm until the silicon nitride film is exposed. According to oneexample, the filling oxide film is formed at a thickness of 6000-12000 Åusing an atmospheric pressure chemical vapor deposition (APCVD) methodor a subatmospheric chemical vapor deposition (SACVD) method.

By way of example, prior to the step of forming the silicon nitridefilm, a pad oxide film may be formed at a thickness of 100-300 Å on theentire surface of the semiconductor substrate.

Although certain example methods and semiconductor devices are disclosedherein, the scope of coverage of this patent is not limited thereto. Onthe contrary, this patent covers every apparatus, method and article ofmanufacture fairly falling within the scope of the appended claimseither literally or under the doctrine of equivalents.

1. A method of forming a trench in semiconductor device, the methodcomprising: forming sequentially a pad oxide film and a silicon nitridefilm on an entire surface of a semiconductor substrate; forming thetrench by etching the silicon nitride film and the semiconductorsubstrate up to a predetermined depth; forming a liner oxide film with athickness thinner than that of the silicon nitride film on an inner wallof the trench; applying a negative voltage to a back surface of thesemiconductor substrate; and forming an insulation film to fill thetrench on the liner oxide film.
 2. A method as defined by claim 1,wherein applying the negative voltage includes applying a voltage of−2000V to −1000V to the back surface of the semiconductor substrate. 3.A method as defined by claim 1, wherein applying the negative voltageincludes using one of a method of applying the negative voltage to theback surface of the semiconductor substrate using an electrostatic chuck(ESC) and a method of injecting electrons into the back surface of thesemiconductor substrate using an electron gun.
 4. A method as defined byclaim 1, wherein the silicon nitride film is formed at a thickness ofabout 1000-3000 Å.
 5. A method as defined by claim 1, wherein the lineroxide film is formed at a thickness of about 100-500 Å by a thermaloxidation process.
 6. A method as defined by claim 1, wherein formingthe insulation film includes forming a filling oxide film to fill thetrench on an entire surface of the silicon nitride film and thenchemical mechanical polishing the filling oxide film until the siliconnitride film is exposed.
 7. A method as defined by claim 6, wherein thefilling oxide film is formed by one of an atmospheric pressure chemicalvapor deposition (APCVD) method and a subatmospheric chemical vapordeposition (SACVD) method.
 8. A method as defined by claim 6, whereinthe filling oxide film is formed at a thickness of about 6000-12000 Å.9. A semiconductor device, comprising: a semiconductor substrate havinga trench of a predetermined depth; a pad oxide and a silicon nitridefilm sequentially formed on an entire surface of the semiconductorsubstrate except for the trench; a liner oxide film formed at an innerwall of the trench and having a thickness thinner than the siliconnitride film; and an insulation film formed on the liner oxide film forfilling the trench, wherein the insulation film is deposited such thatthe trench is filled, under a state where charges stored in the lineroxide film are more than those stored in the silicon nitride film by anapplication of a negative voltage to the back surface of thesemiconductor substrate.
 10. A semiconductor device as defined by claim9, wherein the application of the negative voltage to the back surfaceof the semiconductor substrate is performed by one of electron injectionusing an electrostatic chuck and an electron gun.
 11. A semiconductordevice as defined by claim 9, wherein the negative voltage is −2000V to−1000V.
 12. A semiconductor device as defined by claim 9, wherein thesilicon nitride film is formed at a thickness of about 1000-3000 Å. 13.A semiconductor device as defined by claim 9, wherein the liner oxidefilm is formed at a thickness of about 100-500 Å by a thermal diffusionprocess.